Window sill extension



FIG. 1 is a perspective view of a window sill extension showing my new design;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a top view thereof;

FIG. 4 is a left side elevational view with the right side being a mirror image thereof;

FIG. 5 is a rear view thereof; and,

FIG. 6 is a bottom view thereof.

The broken line showing of a portion of a window sill in FIGS. 2 and 4, a pot and plant in FIG. 2, and a fastener in FIG. 4 is for illustrative purposes only and forms no part of the claimed design. 

The ornamental design for a window sill extension, as shown and described. 